Conventionally, in the contact pressure measurement for soft materials, there have been no measuring instruments which are accurate and have a reproducibility. This is because the state and phenomenon of soft surfaces at the time of contact cannot be comprehended, for example. Also, calibration method for confirming their accuracy has not actually been established yet, thus making it impossible to judge whether each measuring instrument is good or bad.
For example, since strain gauge sensor system is a flat solid sensor, it has detected a bending stress even when it is simply attached to the surface of a site in a curved surface of a human body, and has yielded errors due to surface movement and shifting therebetween when they come into contact with each other and due to hindrance to each other upon expansion and compression. On the other hand, electric resistance system, which measures changes in electric resistance upon compression of an electrically conductive material, is disadvantageous in that its material is uneven, restoration after compression is slow, and reaction, i.e., resolution, upon strong compression is rough. There have been cases where this system is used with units indicated for these characteristics.
In liquid-pressure equilibration system, which is likely to be considered similar to the present invention, a bag in a pressure-receiving section is packed with a liquid. Here, zero correction is necessary for a pressure sensor, which is connected thereto via a tube, as well as its altitude. If their relative height with respect to each other changes after the correction, an error will occur. Also, it is unsuitable for dynamic measurement. It is because load generated upon acceleration yields an error with respect to the weight of the packed liquid.
Even in the case of a pressure-receiving sensor in which a bag is packed with air as with the present invention, when the bag uses an elastic material, the area of the surface to be measured always changes according to a contact strength, thus failing to achieve comparative measurement. In addition, commonly among the above-mentioned measurement systems, there is a pressure-receiving method in which the pressure-receiving section is formed like interconnected sheets or lattices so as to cover the contact surface. In this case, the soft surface may be restrained from changing its curvature or expanding and compressing itself, thereby yielding a measured value different from the actual contact state.